Gain equalizers are important components in fiber optical communication systems. One of the main functions of an optical gain equalizer is to bring all of the separate wavelength signals in a data transmission to the same amplitude, in order to optimize system spectral power budget. The functional requirements of an optical gain equalizer for use in such a system are that it should vary the intensity of the light transmitted as a function of the wavelength of the light, and without appreciably altering the spatial, temporal, or polarization distribution of the light beam.
Many types of optical gain equalizers have been described in the prior art. In U.S. Pat. No. 6,144,488, to H. Okuno, for “Optically Amplifying Device with Gain Equalizing Function”, there is described a semiconductor optical amplifier, in which the gain for each wavelength is varied to provide gain equalization functionality. In U.S. Pat. No. 6,034,812, to T. Naito, for “Gain Equalizer and Optical Transmission System having the Gain Equalizer” there is described a system including three cascaded gain equalizers, the first having a maximum loss at or near the optical amplifier's peak gain, and the others having periodic loss characteristics, arranged such that they fall at or near a wavelength giving one of two gain peaks remaining when the gain characteristic of the optical amplifier the has been equalized by the first equalizer only.
In co-pending Israel Patent Application No. 142,773, hereby incorporated by reference in its entirety, there is described a multichannel gain equalizer utilizing an array of electronically variable optical attenuators. The input optical signal, composed of a number of separate signals, each at its own characteristic wavelength. The input signal is input into a demultiplexer, which separates the individual wavelength components of the signal into n separate channels, λ1, λ2, λ3 . . . λn, one for each wavelength range. Such a demultiplexer is typically constructed of a dispersive grating. Each of these channels is input into its own variable optical attenuator, VOA1, VOA2, VOA3, . . . VOAn. The levels of the signals in each channel 1, 2, 3, . . . n, are detected by means of in-line signal detector elements, and a feedback signal from each detector element is used to control the level of attenuation of each VOA. The resulting signals from all of the separate channels are thus brought to the same level, and are recombined in a multiplexer unit, into a multi-channel, gain-equalized, output signal.
Though the last described gain equalizer may be simpler in design, and, when constructed on an integrated optics substrate using an array of integrated variable optical attenuators, more compact than many other prior art gain equalizers, it still consists of a significant number of components, thereby making its construction comparatively complex.
Gain equalizers, since they are operative to manipulate the various wavelength bands of the light signal to be equalized, generally utilize a wavelength selective filter device for fulfilling their function. In U.S. Pat. No. 5,841,583 for “Multi-path interference filter” and in U.S. Pat. No. 6,046,854 for “Multi-path interference filter with reflective substrate”, both to V.A.Bhagavatula and both hereby incorporated by reference in their entirety, there are described filter devices based on interference effects between different parts of the cross section of a beam which traverse different optical paths, thereby generating the desired phase difference between them.
Two types of filter are described in the Bhagavatula patents. In the first type, the phase shifts are generated by means of an optical path length difference generator formed either by a stack of stepped reflective surfaces that are spaced apart in the direction of beam propagation by at least one nominal wavelength, for varying the physical path lengths between the different parts of the cross section of the beam, or alternatively, in a transmissive version, by using a stepped spacer plate instead of the differently spaced reflective surfaces to generate the path difference. This first type has the disadvantage that the stepped reflective surfaces or the spacer plate are fixed, and no dynamic control of the filtering function is therefore possible. The spectral profile of the filter is thus fixed at the shape defined by the steps used in the filter's construction.
In the second type, the optical path length generator includes a spacer section that is divided into elements having different refractive indices for varying the optical path lengths of the different parts of the cross section of the beam. This type is described in a cylindrical form that can be part of a single mode optical fiber. According to this embodiment, dynamic tuning of the filter's spectral response may be accomplished by changing the optical path lengths by means of electro-optic effects, which vary the refractive index of the material along the optical path length. A controlling electric field is applied by means of jacket electrodes enveloping the electro-optical materials in the optical path. Liquid crystal materials are suggested as a suitable electro-optical material for this purpose.
However, there are two disadvantages to this embodiment. Firstly, it is difficult to implement the geometry proposed, whereby the liquid crystal material is contained in bulk form along the length of the optical path of the axial device. Liquid crystals are usually provided as thin planar layer devices, sandwiched between two electrode-coated cover glasses. Secondly, in the geometry shown, since control is performed from the outside of the device, enveloping the whole of the device, only one of the two interfering optical paths can be made of material which is dynamically controllable at any time, and control of the spectral profile of the filter is thus limited.
There thus exists an important need for an electronically controllable optical filter device, of simple construction and of wide spectral versatility. There also exists a need for an electronically controllable optical gain equalizer of simple construction which can perform spectral signal processing on an optical signal input to it, including dynamic gain equalization.
The disclosures of each of the publications mentioned in this section and in the other sections of the specification, are hereby incorporated by reference, each in its entirety.